This study is being performed in order to identify the factors influencing HIV-1 transmission and to ascertain the time point when transmission of the virus occurs between mother and infant. The Pediatric AIDS Foundation will establish a cohort of maternal/infant pairs; these pairs will be followed from the first or second trimester of pregnancy until 18 months post delivery. Studies will concentrate on virologic and immunologic factors, and will be performed by a consortium of investigators participating in the Ariel Project.